This invention relates to an ink composition for forming a thin metal oxide film on a substrate made of glass, ceramic or the like.
It is known to form a thin metal oxide film on a substrate with an ink containing a base metal compound such as metal oxide hydrate or metal alkoxide, a binder such as resin, a solvent and other additives such as pigment and the like.
For example, JP-A-63-48372 discloses an ink composition containing an organic coloring material, a binder such as silicon alkoxide, and a solvent such as alkyl alcohol.
JP-A-64-56776 discloses a heat-resistant ink for an ink jet printer. This ink contains a hydrolyzed metal alkoxide in the form of solid, a solvent, a binder and an electric conductivity imparting agent.
JP-A-2-60974 discloses an ink composition containing a compound containing a base metal such as metal alkoxide, abletic acid as a viscosity-increasing agent, and a solvent such as butylcarbitol.
It is known to form a thin film on a substrate by screen printing, gravure, intaglio printing, stamping or spraying. For example, U.S. Pat. No. 5,108,479 discloses a method for forming a functional film on a glass plate by screen printing with an ink composition containing a metal alkoxide and a viscosity-increasing agent such as ethyl cellulose or nitrocellulose.
However, as is mentioned hereinabove, when a simple metal alkoxide is contained in an ink composition, it is necessary to hydrolyze the metal alkoxide to form a sol through polycondensation. With this, viscosity of the ink tends to change and its pot life tends to be shortened. When a conventional cellulose type viscosity-increasing agent is contained in an ink, a large amount of carbon tends to remain in a thin film even after the drying and baking of the thin film. Therefore, the thin film tends to have a yellowish brown color damaging its external appearance. Furthermore, its strength such as scratch resistance tends to be substantially lowered.
In general, an ink is applied by printing to a substrate to form a thin film thereon, and then the printed film is baked at a certain high temperature to transform the printed film to a metal oxide film. However, in some cases, the printed film is provisionally dried at a certain temperature which is lower than the baking temperature to obtain a semifinished product, and then this semifinished product is stored until its baking. In this case, the thus dried film made from a conventional ink containing a film-forming component such as simple metal alkoxide and a conventional cellulose type viscosity-increasing agent may have a decomposition of the film-forming component due to moisture in the atmosphere or the like, and thus the baked film may have a refractive index which is substantially lower than the theoretical value expected from the ink composition or may become inferior in durability.
U.S. Pat. No. 5,338,350 discloses an ink composition comprising: one selected from the group consisting of a halogen-containing metal alkoxide, a mixture of the halogen-containing metal alkoxide and another metal alkoxide, and a mixture of the halogen-containing metal alkoxide and a metal oxide sol; nitrocellulose H60 according to Japanese Industrial Standard K 6703; and at least one selected from the group consisting of ethylcarbitol and butylcarbitol. The ink composition disclosed in this patent has a viscosity of about 23.2 poises (see column 3, lines 55-56).